1. Field of the Invention
The present invention relates to a cathode ray tube, more particularly, to a structure of an electron gun for enhancing resolution of a cathode ray tube.
2. Discussion of the Related Art
FIG. 1 is a diagram illustrating the structure of a color cathode ray tube of the related art.
Referring to FIG. 1, the color cathode ray tube of the related art includes a front side glass panel 1, and a rear side glass funnel 2 connected to the panel 1. The panel 1 and the funnel 2 are connected to each other in a manner that their inside is in a vacuum state, thereby forming a vacuum tube. Inside surface of the panel 1 is a fluorescent screen 11, and an electron gun 8 is housed in the funnel 2 on the opposite side of the fluorescent screen 11.
A shadow mask 3 with an electron beam color selecting function is situated at a predetermined distance from the fluorescent screen 11, and the shadow mask 3 is coupled with a mask frame 4.
Also, the mask frame 4, which is connected to a mask spring 5, is connected to a stud pin 6 to be supported to the panel 1.
The mask frame 4 is jointed with an inner shield 7 made of magnetic material to reduce the movement of an electron beam 5 caused by an external magnetic field. Accordingly, the effect of a geomagnetic field at the rear side of the cathode ray tube is reduced.
On the other hand, a convergence purity magnet (CPM) 10 for adjusting R, G and B electron beams emitted from the electron gun 8 to be converged on one spot, and a deflection yoke 9 for deflecting the electron beams are mounted on a neck portion of the funnel.
Also, a reinforcing band 12 is used to reinforce the front surface glass under the influence of a high interval vacuum state of the tube.
To briefly explain how the color cathode ray tube with the above construction operates, the electron beams emitted from the electron gun 8 are deflected in the horizontal and vertical directions by the deflection yoke 9, and the horizontally/vertically deflected electron beams pass through a beam passing hole on the shadow mask 3 and eventually strike the fluorescent screen 11, thereby displaying a desired image.
FIG. 2 depicts the structure of an electron gun of the related art.
As illustrated in FIG. 2, the electron gun 8 of the related art can largely be divided into three parts: a triode unit, a main lens, and a pre-focus lens between the triode unit and the main lens.
The triode unit includes a cathode 21 having a built-in heater 20, a control electrode 22 for controlling electron beams emitted from the cathode 21, and an accelerating electrode 23 for accelerating the electron beams, in which the cathode 21, the control electrode 22, and the accelerating electrode 23 are arranged in-line.
The main lens includes a main focus electrode 26 and an anode 27 for focusing electron beams generated from the triode unit and accelerating the electron beams in the end. More specifically, the main focus electrode 26 includes a cap electrode 261 having a race track shaped rim portion, and an electrostatic field control electrode 262. The anode 27 includes a cup electrode 271 having a race track shaped rip portion and an electrostatic field control electrode 272. Here, the electrostatic field control electrodes 262 and 272 are to equalize convergence force of three electron beams, and recessed to a certain direction from the cap electrode 261 or the cup electrode 271. FIG. 3 illustrates the anode 27 seen in D direction of Fig. 1, and FIG. 4 illustrates the main focus electrode 26 seen in C direction of FIG. 1.
The pre-focus lens includes a first pre-focus electrode 24 and a plate-shaped second pre-focus electrode 25.
The control electrode 22 is earthed. A voltage of 500–1000V is applied to the accelerating electrode 23 while a high voltage of 25–35 KV is applied to the anode 27. An intermediate voltage, e.g., 20–30% of the applied voltage to the anode 27, is applied to the main focus electrode 26.
When a designated voltage is applied to each of the electrodes of the electron gun 8, the electron beams generated at the triode unit are focused and accelerated, and later strike the fluorescent screen 11.
In general, for a cathode ray tube using an in-line electron gun, Red, Green and Blue electron beams are aligned horizontally. Thus a self-convergence type deflection yoke 9 that converges three electron beams to one spot is usually used.
As shown in FIG. 5, the self-convergence type deflection yoke 9 makes a horizontal deflection magnetic field (HB) in a pin-cushion shape, and a vertical deflection magnetic field (VB) in a barrel shape, resulting in the prevention of a mis-convergence problem on the fluorescent screen 11.
The magnetic fields can be categorized into diode and tetrode magnetic fields. The diode magnetic field deflects electron beams in horizontal and vertical directions. On the other hand, the tetrode magnetic field converges electron beams in the vertical direction and diverges in the horizontal direction, thereby causing astigmatism. In result, the shape of the electron beam spot is distorted and focusing characteristics thereof are deteriorated.
To elaborate the above phenomenon with reference to FIG. 11, although the magnetic field is almost perfectly uniform, an astigmatism phenomenon occurs to the electron beams in a peripheral portion of the fluorescent screen 11 (i.e. a peripheral portion of the screen) by a minute pin-cushion or barrel magnetic field component. Therefore, the shape of the electron beam spot is distorted and focusing characteristics thereof are deteriorated.
More specifically, a deflection magnetic field is not applied to the central portion of the fluorescent screen 11, so the electron beam spot has a circular shape. In the peripheral portion of the fluorescent screen 11, however, the electron beams are diverged in the horizontal (H) direction and overly converged in the vertical (V) direction, causing a low-density haze phenomenon to a high-density horizontally elongated core and the upper and lower parts of the core. Especially, deterioration in the resolution is worse at the peripheral portion of the screen. This problem gets worse for large cathode ray tubes and great deflection angles.
Basically the haze phenomenon at the peripheral portion of the screen occurs because the influence of deflection aberration is greater at the center of the deflection yoke 9. For example, the electron beams in the horizontal direction are almost circular because the divergence force of the deflection magnetic field and the convergence force by a distance difference are cancelled out or counterbalanced with each other. On the contrary, in the vertical direction the convergence force by the deflection aberration and the convergence force by the distance difference are superposed, resulting in the occurrence of the haze phenomenon.
Therefore, to get rid of the haze phenomenon, the triode unit should be adjusted properly.
FIG. 6 illustrates a control electrode in an electron gun of the related art.
Referring to FIG. 6, an electron beam passing hole 221 of the control electrode 22 has a circular shape, and the diameter of the passing hole is about 0.5 mm–0.7 mm. The thickness of the electrode around the electron beam passing hole 221 ranges from 0.08 mm to 0.1 mm.
Now referring to an accelerating electrode 23 in FIG. 7, there is a slot 232 formed on the circumference of each electron beam passing hole 231. More specifically, the slot 232 is formed on the opposite side of a first pre-focus electrode 24 (shown in FIG. 8), and the shape of the electron beam passing hole 231 is a circle or square. The thickness of the accelerating electrode 23 is approximately 0.37 mm, and the depth of the slot 232 is approximately 0.15 mm, which is about 40% of the entire thickness of the accelerating electrode 23. Also, the slot 232 is horizontally elongated, that is, the horizontal size of the slot 232 is greater than the vertical size thereof. This horizontally elongated slot 232 serves to reduce the haze phenomenon at the peripheral portion of the screen.
FIG. 8 illustrates a first pre-focus electrode 24. The diameter of an electron beam passing hole 241 of the first pre-focus electrode 24 ranges from 0.9 mm to 1.5 mm.
FIG. 9 illustrates a second pre-focus electrode 25. The second pre-focus electrode 25 is in a plate shape, and the diameter of an electron beam passing hole 251 thereof ranges from 3.0 mm to 4.0 mm. In some cases, the second pre-focus electrode 25 takes a cap or cup shape. Because an applied voltage to the second pre-focus electrode 25 is low, a pre-focus lens is formed around the second pre-focus electrode 25.
As shown in FIG. 10, the size of an electron beam incident on a main lens, Db, is determined by divergence angle of an electron beam generated at the triode unit and by convergence force of the pre-focus lens. In FIG. 10, Db (H) indicates a horizontal size of the electron beam, and Db (V) indicates a vertical size of the electron beam.
In general, among other design characteristics of an electron gun 8, lens magnification, repulsive space charge (electric force), and spherical aberration of the main lens are major factors that influence spot size of an electron beam formed on the fluorescent screen 11.
The lens magnification actually does not have much effect on the spot size (Dx) and its utility as a design element of the electron gun is very low because there are several fixed conditions like a voltage, a focal length, and a length of the electron gun.
On the other hand, the influence of the repulsive space charge force on the spot size (Dst) indicates a phenomenon that the spot size (Dst) is enlarged due to the repulsion and the collision between electrons in the electron beam. To obviate such phenomenon, a special designing is needed to increase an angle to which the electron beams travel (hereinafter, it is referred to as ‘emission angle’).
The influence of the spherical aberration of the main lens on the spot size (Dic) indicates a phenomenon that the spot size (Dic) is enlarged due to the difference between focal lengths of an electron that passed through a short axis of the lens and an electron that passed through a long axis of the lens. Unlike the repulsive space charge force, if the beam emission angle on the main lens is small, the spot size on the fluorescent screen 15 can be reduced.
To summarize the above, the spot size (Dt) on the fluorescent screen 15 can be expressed as follows:Dt=√{square root over ((Dx+Dst)2+Dic2)}
When it comes to the electron gun of the related art, the size (Db) of an electron beam incident on the main lens is approximately 2.5 mm–3.0 mm. When Db is greater than the range, the spot size is increased due to spherical aberration, and when Db is less than the range, the spot size is again increased due to repulsive space charge (electric) force.
As shown in FIG. 11, in the electron gun of the related art, the haze phenomenon is more prevalent in the vertical direction as it gets closer to the peripheral portion of the screen. To suppress this phenomenon, a slot is formed on an accelerating electrode 23 as illustrated in FIG. 12.
As the slot of the accelerating electrode 23 is deeper, an electron beam incident on the main lens is horizontally elongated, reducing a vertical size of the electron beam. As a result, the influence of deflection aberration is lessened, and the haze phenomenon at the peripheral portion of the screen is suppressed. Meanwhile, repulsive space charge (electric) force is increased, and thus the vertical size of the electron beam is increased. Accordingly, vertically elongated beam spots are created at the central portion of the screen, and spots at the peripheral portion of the screen are less influenced by the haze phenomenon.
However, the above schemes are not sufficient to obtain a satisfactory resolution at the peripheral portion of the screen. Therefore, to manufacture a cathode ray tube having a high resolution, a dynamic voltage with a parabolic waveform is applied, as shown in FIG. 13, to form a dynamic quadrupole lens (DQ lens) as shown in FIG. 14.
However, to apply the dynamic voltage, a separate circuit is needed. This consequently raises the manufacture cost of an electron gun, and lowers price competitiveness of a cathode ray tube.